Explanation:

Correct Answer is (b).

There are two options- A and E- as the potential first statement. Chronologically speaking, Statement E refers to a time period before the one stated in A and should logically precede A. Thus, Statement E is the first statement:

E and C:
E. World leaders did well to avoid protectionist trade policies in the aftermath of the Great Recession of 2008.
C. After all, they had learned their lessons from the global trade war of the 1930s which deepened and prolonged the Great Depression, or so it was thought.
C and A: A introduces a contradiction here stating how Trump has recently announced imposition of tariffs-
C. After all, they had learned their lessons from the global trade war of the 1930s which deepened and prolonged the Great Depression, or so it was thought.
A. American President Donald Trump last week announced that his administration would soon impose tariffs on the import of steel and aluminium into the U.S. for an indefinite period of time.

A and B: B states How The EU has reacted to it-
A. American President Donald Trump last week announced that his administration would soon impose tariffs on the import of steel and aluminium into the U.S. for an indefinite period of time.
B. The European Union, one of the largest trading partners of the U.S., has since vowed to return the favour through retaliatory measures targeting American exporters.

B and D:
B. The European Union, one of the largest trading partners of the U.S., has since vowed to return the favour through retaliatory measures targeting American exporters.
D. The EU is expected to come out with a list of over 100 items imported from the U.S. that will be subject to scrutiny.

The correct sequence is E-C-A-B-D